As an example of a speed change transmission apparatus operable to effect speed change operations on a drive force from an engine, with using a stepless speed change device, a planetary transmission mechanism and a clutch, there is one known from Patent Document 1.
The speed change transmission apparatus disclosed in Patent Document 1 includes: a hydraulic unit 4 receiving drive force from an engine 1 via a first gearwheel transmission 3; and a summator planetary gear 10 operable to receive an output from the hydraulic unit 4 via a second gearwheel transmission 5 and a first input shaft 6 and operable also to receive directly the drive force from the engine 1 not via the hydraulic unit 4, but via a third gearwheel transmission 7 and a second input shaft 8. The apparatus further includes: a first planetary gearstep 12 and a second planetary gearstep 13 which receive force from the summator planetary gear 10 via clutches K1, K2; and an output shaft 14 coupled to a planetary carrier shift 25 and coupled to either the planetary carrier shaft 25 or a sun wheel shaft 30 via clutches K3, K4. The summator planetary gear 10 includes a first planetary line including a first sun wheel 20 and a second planetary line including a second sun wheel 23. The first planetary gearstep 12 includes a clutch KR and the second gearstep 13 includes a clutch KV. With a speed change operation effected on the hydraulic unit 14 and subsequent appropriate operations of the respective clutches KV, KR, K1, K2, K3, K4, the engine drive force is changed in four speed steps and is changed steplessly in each speed step.
Patent Document 1: U.S. Pat. No. 5,911,645 (columns 3-6, FIGS. 1-3).
When the above-described conventional speed change apparatus is employed, it is necessary to provide great numbers of planetary transmission mechanisms and clutches, thus tending to invite disadvantage in terms of construction.